In one type of installation of the above kind, cold air is drawn from outside the interior (i.e. the passenger compartment or cab) of the vehicle, optionally through an air conditioning unit, into the interior of the vehicle after being heated if necessary. The treated air is admitted to the interior of the vehicle through various inlet ports situated in particular locations. Commonly, cold or heated air can be admitted in the form of a forced draught set up by a forced draught unit including a variable speed rotary device such as a fan or blower.
The control system for such an installation essentially includes two control members which are adapted to regulate the temperature and the distribution, respectively, of the air to be passed into the interior of the vehicle. A control member for the forced draught unit, and another control member for the air conditioning unit (if the vehicle is equipped with such an option) are also generally provided. This kind of control system, in which the above mentioned control members consist of knobs, sliding cursors, levers etc., are usually mounted on the fascia panel of the vehicle.
Such known arrangements have the disadvantage that they do occupy space on the fascia panel, which already carries other controls and instruments. In addition, control systems of this type do not lend themselves to a particularly compact grouping of the control members such as levers and so on. There is a tendency to give rise to interactions between the various control members, for example between the heating control and the air distribution control.